General Assembly, considered one of Spectacular Bid's most formidable rivals for this spring's Triple Crown races, lost his 1979 debut as a 1-to-10 favorite at Hialeah today.

The son of Secretariat was defeated by a head by unheralded Coup De Chance in a minor allowance race, but his jockey, Angel Cordero Jr., had to share the blame.

Over a Hialeah racing strip where horses on the rail have an almost unbeatable advantage, Cordero rode as if nothing could beat him. He restrained his mount as he came out of the gate and kept him wide as he cruised toward the leaders on the backstretch. While he was biding his time, J. P. Souter was hustling 7-to-1 shot Coup De Chance to the front along the rail.

As the field reached the turn, General Assembly drew abreast of the leader and looked for a moment as if he were going to swoop to an easy victory. But Coup De Chance, racing on the best part of the track, responded without his jockey using the whip. Cordero hit General Assembly 17 times through the stretch, in vain.

The horses ran the seven furlongs in 1:22 2/5, rather unimpressive time considering that $20,000 claiming animals had covered the distance only four-fifths of a second slower earlier this afternoon.

Cordero seemed unperturbed by the defeat.

"He ran a good race," the jockey said. "He's been away a long time. He's a big colt and he's still growing."

Cordero undoubtedly would love to ride General Assembly to a victory over Spectacular Bid, since the latter's trainer, Bud Delp, has spent the last three days blasting him. Delp accused Cordero of trying to hurt Spectacular Bid on the first turn of the Florida Derby, cutting him off with his mount, Fantasy 'N Reality.

"I've never met Cordero," Delp said. "I wouldn't know him if he were climbing a banana tree. But he could have jeopardized a $20 million horse. If I had been riding Spectacular Bid I would have nudged his horse and Cordero would have been sent flying into the stands."

Even though Cordero has diverted Delp's wrath from Ron Franklin, that jockey's future on Spectacular Bid appears dim. Delp has said that he will announce owner Harry Meyerhoff's decision on a rider Monday morning. Besides Franklin, the candidates are Bill Shoemaker, Darrell McHargue and Jacinto Vasquez.